Add transaction fee estimation helper in transaction package

[Cjay-Cyber-2]

Description — what to implement and the why/impact.

Add a transaction fee estimation helper in @astroid/transaction to calculate expected Stellar network fees prior to submitting agent transactions.

Context & Requirements — background, constraints, design references, edge cases;

Background: Autonomous agents need to verify they have sufficient funds for network transaction fees before building and signing operations.
Constraints: Must handle Stellar fee bumps and base fee calculation accurately.

Acceptance Criteria — a checklist ("- [ ] ...") of specific, testable conditions that define "done".

• Fee estimation function exported from @astroid/transaction
• Handles standard transaction envelopes and multi-operation transactions
• Comprehensive unit tests verifying fee calculations against mock Stellar ledger data

Implementation Guidance — likely files/modules to touch (use real paths from the repo context)

Inspect packages/transaction/src/ and integrate with existing transaction building utilities.

Testing & Validation — how the contributor should prove it works

Run pnpm test inside packages/transaction to confirm unit tests pass successfully.

Submission Guidelines — must open a PR that includes "Closes #"; assignment is required before starting.

---

Wave complexity: Medium